SiC whisker and particle reinforced ceramic cutting tool material

Abstract: According to the present invention there is now provided a ceramic cutting tool material comprising a ceramic, preferably aluminium oxide based, matrix with 5-50, preferably 10-40 % by volume of homogeneously dispersed SiC-whiskers and 1-20, preferably 3-8 % by volume of particulate additions with a size <500 nm, preferably <200 nm, based upon carbides and/or nitrides and/or borides of metals from group Vb(V,Nb,Ta) in the periodic system or solid solutions thereof. The nanosize particulate additions are predominantly present inside the matrix grains and not in the grain boundaries.

Description: The present invention relates to ceramic cutting tool materials and particularly to such cutting tool materials with a ceramic matrix in which monocrystalline whiskers (hair crystals) of silicon carbide and small (significantly less than 1 µm) particles of carbides, nitrides and/or borides of metals from group VB(V,Nb,Ta) are uniformly distributed resulting in an increased strength and toughness without negatively influencing the wear resistance of the material. Ceramic cutting tool materials have now been available for several decades but they have not until recently had any great commercial importance for use in chipforming machining. The main reason for the limited growth of ceramic cutting tools has been sudden and unexpected tool failures because of their inherent inadequate strength and toughness. In recent years, the properties of ceramic cutting tool materials have been improved in many respects and their use in cutting of cast iron and heat-resistant alloys (e.g. nickel-base alloys) has relatively increased. Aluminium oxide based cutting tool materials are very sensitive to thermal crack formation because aluminium oxide in itself has a relatively poor thermal conductivity. This leads to very short tool lives in machining steel, particularly under conditions with short operating times and varying cutting depth. To a certain extent, the thermal properties have been improved by particulate additions of titanium carbide and/or titanium nitride which enhance the thermal conductivity of the tool material. The addition of titanium carbide/nitride also increases the hardness of the material. In comparison with pure aluminium oxide materials, an increased tool life is therefore obtained in the cutting of harder work piece materials and in operations demanding thermal shock resistance. However, this kind of material has too poor a toughness behaviour for a more general use in the cutting of steel. A later development relates to alloying of uniformly dispersed fine-grained zirconium oxide particles in a matrix of aluminium oxide. A transformation of the 'metastable' zirconium oxide particles during use increases both strength and toughness and thus leads to a more predictable tool life. The thermal properties of said type of materials are, however, only slightly better than those of pure aluminium oxide materials. Therefore, initation and growth of thermally induced cracks is still a great problem in practical cutting operations generating high cutting edge temperatures such as cutting of steel. It has recently been shown that alloying of SiC-whiskers, with mono-crystalline hair crystals, in a matrix of aluminium oxide leads to a greatly improved fracture toughness and strength. Ceramic cutting tool materials based upon said concept have shown very good performance in the cutting of heat-resistant materials in particular. It is well known that particulate additions can be used to improve the properties of a brittle ceramic material, e.g., USP 4,320,203 which relates to additions of TiN and Ti(C,N). Depending on the nature of the additions the operating toughening mechanisms can be crack deflection, microcracking, transformation toughening or crack bridging. It is characteristic for these particulate additions that the size of the particles are of the same order of magnitude as the matrix material e.g. in the order of 1-5 µm and that they are located in the grain boundaries of the matrix material. Only in the case of ZrO 2 -additions a smaller grain size than the matrix is needed in order to suppress a transformation during fabrication of the material. However, the zirconia particles are still predominantly located in the grain boundaries of the alumina matrix material. It is also characteristic for the particulate reinforced materials that property improvements are achieved up to rather high particulate contents, normally up to 15-30 percent by volume. It has recently been found that further improvements of the properties are possible in the silicon carbide whisker-reinforced cutting tool materials, especially the strength, if small additions of nano-size particles are added to the whisker reinforced materials. In USP 5,123,935 such a material is described containing Ti-compounds of nanosize. It is believed that the function of the small Ti-compounds is to strengthen the alumina matrix and to refine the microstructure. US-A-4,507,224 relates to ceramics containing fibers of silicon carbide. It discloses a ceramic material comprising a ceramic matrix, e.g. alumina, with 5-50 % SiC whiskers and 2-20 % of particulate additions with a size as low as 200 nm, e.g. TiN. It has now surprisingly been found that further improvements are possible if the added nano-sized particles have a much lower thermal expansion coefficient than the matrix material i.e. alumina. The thermal expansion coefficient for alumina is 8.4x10-6 K -1 (300-1300 K). Reported values for TiC (7.4-8.6) are slightly lower than for alumina and reported values for TiN (8.0-9.3) slightly higher. If small nano-particles with much lower thermal expansion coefficients like TaN (5.0), TaC (4-6.5) and TaB 2 (5.5) are added higher strengths are obtained which is believed to be caused by a more effective grain refinement. This could be caused by the fairly large internal stresses that are created within the alumina grains upon cooling from the sintering temperature. The internal stresses themselves can also have a strengthening effect due to interaction with an external stress field. The ceramic cutting tool material according to the present invention comprises an oxide-based, preferably aluminium oxide based, matrix with 5-50, preferably 20-40 % by volume of homogeneously dispersed whiskers based upon silicon carbide and 1-20, preferably 3-8 % by volume of particulate additions with a size <200 nm, based upon carbides, nitrides and/or borides of metals from group VB(V,Nb,Ta) preferably TaC, TaN, NbC, or VC or solid solutions thereof. Said whiskers shall consist of monocrystals with a diameter of 0.5 - 10 µm and a length of 2.5 - 100 µm with a length/diameter ratio preferably of 5 - 10. The whiskers may at least partly be replaced by monocrystal platelets with a diameter of 5 - 20 µm and a thickness of 0.5 - 4 µm. Said matrix shall consist essentially of ceramic oxides, preferably alumina, or of ceramic oxides mixed with hard carbides and/or nitrides and/or borides and/or binder metal. The grain size of the matrix shall be < 10 µm, preferably < 4 µm. The nano-size compounds may contain oxygen up to 5 weight % and may be stoichiometric as well as nonstoichiometric. The material may further contain zirconium oxide particles up to 20 weight %, which may further increase strength and fracture toughness. The invention has been described with reference to an oxide based matrix. However, the same favourable results may also be obtained for a nitride-based matrix such as Si 3 N 4 . The cutting material according to the invention is made by wet milling and mixing of matrix powder, nanosize powder, whisker and/or platelets and conventional sintering aids. After drying the mixture is pressed to desired geometrical form and sintered to near theoretical density, and after the sintering the possible remaining porosity can be further reduced using hot isostatic pressing. If it is not possible to obtain a closed porosity by pressureless sintering the material can be pressure sintered with a suitable graphite tool or after encapsulation be hot isostatically pressed to desired density. The sintering conditions depend upon the raw-material and is chosen so that the material reaches a density which exceeds 98%, preferably 99.5% of theoretical density. Example 1 Alumina, with a grain size of 0.6 µm, are mixed with 25 weight % (which corresponds to 30 volume%) silicon carbide whiskers with a diameter of 0.6 µm. Small (<0.2 µm) particles of TiN and TaN are added in an amount of 5 volume per cent In one experiment a further addition of zirconia is made together with TaN additions. All materials are hot-pressed at 1840°C at 28 MPa for one hour. Small test bars are cut from the hot pressed disc in order to evaluate the transverse rupture strength (TRS) in three-point bending test. The test bars are polished before testing. Fracture toughness (K 1c ) is also evaluated by means of a so called indentation method. In said method an indentation is made by means of a pyramid diamond tip and fracture toughness is calculated from the size of the cracks produced from the corners of the indentation. Strength and fracture toughness values are given in Table 1. TRS (MPa) K 1c MPam 1/2 1. Al 2 O 3 + 25 weight% SiC-whiskers 850 6.8 2. Al 2 O 3 + 25 weight% SiC-whiskers + 5 vol% TiN 900 6.9 3.Al 2 O 3 + 25 weight% SiC-whiskers + 5 vol% TaN 1100 7.1 4. Al 2 O 3 + 25 weight% SiC-whiskers + 5 vol% TaN + 10 weight% ZrO 2 1045 7.4 From Table 1, it is evident that already small additions of nanosize particles give significant strengthening but fracture toughness is not very much affected. Increased strength means that the risk of tool failure due to overloading of the insert is decreased and consequently the reliability is improved.

Claims: A ceramic cutting tool material comprising a ceramic based matrix with 5-50 % by volume of homogeneously dispersed SiC-whiskers and 1-20 % by volume of particulate additions based upon carbides, nitrides and/or borides of metals from group VB (V,Nb,Ta) in the periodic system or solid solutions thereof, characterized in that said particulate additions have a size < 200 nm and are located inside the matrix grains and not in the grain boundaries.

Inventors: BRANDT GUNNAR, BRANDT, GUNNAR
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 1994
Main classification: C04B35
IPC codes: C04B35